The present invention is directed to bicycle transmissions and, more particularly, to bicycle transmissions internally mounted within a wheel hub.
An example of a conventional internal shifter unit is the shifter unit disclosed in U.S. Pat. No. 5,078,664. An internal shifter unit is usually provided with a clutch that is used to select among a plurality of drive force transmission paths via a gear mechanism equipped with a planet gear or the like provided on the inside. When the shift operation unit attached to the handlebar or the like is operated, the inner cable of the shift cable is pulled or relaxed to operate the clutch. When the inner cable is pulled, the clutch can be operated by this pulling force, but since this pulling force is absent when the cable is relaxed, the energizing force of a return spring or the like is utilized to operate the clutch in the reverse direction.
In the case of an internal shifter unit, when the pedal is forcefully pressed down and a large drive load is applied, a large separation resistance is generated in the gear mechanism inside the internal shifter unit. More specifically, when a drive force is applied, the friction increases at the engaged portion between two gears or between a pawl and a tooth in the transmission, which is a problem in that clutch switching becomes more difficult. The spring force of the return spring can be increased so that the return spring will overcome this separation resistance and allow operation when the cable is relaxed, but then when the cable is pulled it will must pull against this increased spring force of the return spring, so a greater cable pulling force is needed. Thus, if the strength of the return spring is reduced, the operating force with respect to the clutch is weakened during relaxation of the cable, and if the spring strength is increased, a greater force is needed to pull the cable.